1. Field of the Invention
The present invention relates to a grain boundary-insulated semiconductor ceramic, a semiconductor ceramic capacitor, and a method for producing a semiconductor ceramic capacitor. More specifically, the present invention relates to a grain boundary-insulated semiconductor ceramic containing SrTiO3 as a main component and a diffusing agent that contains a grain boundary insulating agent and a glass component, a semiconductor ceramic capacitor including the grain boundary-insulated semiconductor ceramic, and a method for producing the semiconductor ceramic capacitor.
2. Description of the Related Art
Grain boundary-insulated semiconductor ceramic capacitors are capacitors in which the apparent dielectric constant is increased by forming semiconductor crystal grains and insulating crystal grain boundaries, and are widely known as capacitors having small sizes and large capacitances.
For example, Japanese Unexamined Patent Application Publication No. 3-285870 (Patent Literature 1) discloses a grain boundary-insulated semiconductor porcelain composition in which a crystal grain boundary of semiconductor porcelain represented by a general formula (Sr1-x-zBaxCazMy)+TilO3+mN+nZ (where M represents at least one element selected from Nb, Ta, W, and rare-earth elements, N represents at least one element selected from Mn, Al, etc., and Z represents at least one of Pb and B) wherein x, y, z, l, m, and n are each within a predetermined range, is insulated by a compound containing at least one of Cu, Bi, Pb, B, and Si.
In Patent Literature 1, a Bi2O3—Pb3O4—B2O3—CuO—SiO2-based material is used as a grain boundary insulating agent that insulates grain boundaries in the semiconductor porcelain containing SrTiO3 as a main component, and this material is diffused during heat treatment to form a desired grain boundary insulating layer. Thus, a semiconductor ceramic capacitor having a large product of the apparent dielectric constant and the breakdown voltage is obtained.
Japanese Unexamined Patent Application Publication No. 2-111006 (Patent Literature 2) discloses a method for producing a grain boundary-insulated semiconductor porcelain capacitor, the method including steps of preparing a semiconductor porcelain; mixing the semiconductor porcelain with a diffusing agent prepared by adding a glass powder to a grain boundary insulating agent that insulates grain boundaries of the semiconductor porcelain in an amount of 1% by weight to 100% by weight relative to the grain boundary insulating agent, and heat-treating the resulting mixture while stirring to insulate the grain boundaries of the semiconductor porcelain; and forming electrodes by plating on both main surfaces of the semiconductor porcelain whose grain boundaries have been insulated.
In Patent Literature 2, a diffusing agent containing a Bi2O3—PbO—CuO-based grain boundary insulating agent and a Bi2O3—PbO—CuO—B2O3-based glass powder is mixed and stirred with SrTiO3-based semiconductor porcelain to prepare semiconductor porcelain. Thus, variations in electrostatic capacity are reduced.
Recently, the consciousness of environmental issues has been increasing worldwide. In the European Union (EU), in particular, regulation of the use of lead (Pb), which causes environmental pollution, has been provided by, for example, the Restriction of Hazardous Substances (RoHS) that restricts the use of certain hazardous substances in electrical and electronic equipment and the End of Life Vehicles Directive (ELV) that prescribes environmental regulation of end-of-life vehicles. Accordingly, development of materials which are an alternative to Pb-based materials has been urgently required.
However, the Bi2O3—Pb3O4—B2O3—CuO—SiO2 based material of Patent Literature 1 contains Pb in the grain boundary insulating agent. The content of Pb3O4 is small but when the Pb3O4 is removed from the grain boundary insulating agent, the diffusing power of the grain boundary insulating agent decreases during heat treatment and it becomes difficult to sufficiently oxidize crystal grain boundaries. In particular, the grain boundary insulating agent does not sufficiently thermally diffuse in a semiconductor ceramic capacitor having a tubular shape such as a circular tube or a rectangular tube and having a large wall thickness into the inside of the capacitor, which may result in a decrease in electrical properties.
In Patent Literature 2, Pb is used in both the grain boundary insulating agent and the glass powder, and thus the same problem as that in Patent Literature 1 occurs. In addition, boron (B) is contained in the glass component of Patent Literature 2. When B is incorporated in the glass component, although the effect of increasing the diffusing power can be obtained to a certain degree, the insulation resistance and the breakdown voltage tend to decrease, which may result in a decrease in electrical properties.